Benzisoxazoles, formation from oximes

Ring formation by intramolecular condensation of oximes has specific geometrical requirements. The oximes 22 formed from the pentafluorodeoxybenzoins cyclize to 1,2-benzisoxazoles 23 at 140-1 50c C indimethylformamide when the hydroxy group is syn to the pentafluorophcnyl... 

The endothermicity of this reaction is 40 kJ mol1 and so we deduce benzisoxazole has significant aromaticity. No direct measurements of the enthalpy of formation of any acyclic oxime ethers appear in the literature. However, from the calculational literature [69] we find a value for the enthalpy of formation of CH3CH=NOCH3 of -7 kJ mol1, Unmeasured by combustion calorimetry because of experimental complications, the enthalpy of formation of benzisoxazole was recently suggested [70] to be 139 kJ mol-1. 

A route to regioisomeric benzisoxazole structures featuring a key copper-catalyzed C—O bond formation has been reported. In an intramolecular cyclization similar to that shown in Scheme 24.34 for benzoxazoles, benzisoxazoles such as 64 could be synthesized from Z-conflgured o-bromoacetophenone oximes under the action of a diamine-based copper catalyst (Scheme 24.36) [128]. A range of 3-methyl-l,2-benzisoxazoles were synthesized in good yields. 

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